The invention relates to a differential pulse code modulation encoding apparatus comprising an input for receiving the signal to be encoded, a difference circuit for supplying a differential signal formed by a given number of binary elements representing the difference between the signal to be encoded and a prediction signal, a transmission prediction circuit for supplying this prediction signal, a transmission transcoding circuit for reducing the number of binary elements of the differential signal as a function of a control information, an activity measuring circuit for supplying an activity information, a statistic encoding circuit for variable word length coding of the output signal of the transcoding circuit, a buffer memory circuit for storing the output words of the statistic encoding circuit and for supplying them to the output of the apparatus and an occupation measuring circuit for supplying an occupation information of the buffer memory circuit.
An encoding apparatus of this kind is used on a large scale especially in the field of digital television. In this connection, reference may be made to the article by Peter Pirsch entitled "Design of DPCM Quantizers for Video Signals Using Subjective Tests" published in July, 1981, in the magazine "IEEE Transactions on Communications", Vol. Com. 29, No. 7.
It is known that in an apparatus of this kind, the problem is met of overflow of the memory circuit due to the statistic encoding circuit. In fact, it may happen that the encoding of a picture implies the use of words of great length so that the number of data entering the memory circuit is larger than that leaving it.
It is known in order to solve a problem of this kind to operate at the level of the transcoding circuit in such a sense that the statistic encoding circuit supplies words of smaller length (see, for example, the article by Toshio et al entitled: "Statistical Performance Analysis of an Interframe Encoder for Broadcast Television Signals" published in December, 1981, in the magazine "IEEE Transactions on Communications", Vol. Com. 29, No. 12).
However, with this kind of measurements, the picture is then encoded in too rough a form and the quality of the latter becomes poor.